Blow pit and method of operating the same



Feb. 20, 1934. H. w. FRENCH BLOW PIT AND METHOD OE OPERATING THE SAME Filed may' 2e, 1932 4 Sheets-Sheet l INVENTOR Feb. 20, 1934. y H. w. FRENCH -A 1,947,707

BLOW PIT AND METHOD OF OPERATING THE SAME Filed May 26, 1952 Y 4 Sheets-Sheet 2 INVENTOR @ff/7M Feb. 2o, 1934. H. wQ FRENCH BLOW PIT AND METHOD OF OPERATING THE SAME Filed May 25, 1932 4 Sheets-Sheet 3 INVENTOR Feb.'20, 1934. H. w. FRENCH- I BLOW PIT AND METHOD OF' OPERATING THE SAME 4 sheets-sheet 4 Filed May 26, 1932 l INVENTOR ZM y Y 7% C-ATTDRNEY mwN Patent-ed Feb. 20, 1934 UNITED STATES BLOW PIT AND METHOD OF OPERATING THE SAME

Hobert W. French, Erie, Pa.,

assignor to Hammermll Paper Company, Erie, Pa., a corporation Of Pennsylvania Application May 26, 1932. Serial No. 613,671

23 Claims.

f This invention relates to improvements in the construction of blowpits and to improved methods of controlling and Operatingvthe same.

Heretofore it has been diflicult, if at all possible, to maintain uniform conditions in the operation of a blow pit, involving the drainage of pulp discharged from a digester, the washing of the pulp and the removal of the pulp from the pit. Due to the uneven distribution of the pulp normally encountered in blow pit operations and the lack of uniform conditions throughout the pulp system, variable conditions of drainage and variable degrees of washing have taken place through the pulp mass. This has resulted in imperfect removal of the liquor and impurities from the pulp or in an undue lengthening of the period of treatment in the pit. Furthermore, the removal of the pulp after the draining and washing operation has been slow and cumbersome and extremely inemcient. It has been the common practice to remove the pulp by forcing it toward a discharge outlet by means of a powerful stream of water directed at random from a hose.

It has been an important' object of the present invention to provide uniform conditions of operation of a blow pit under all circumstances. By the practice of the invention the pulp in the pit is distributed evenly over the drainage surface and is maintained in such a condition that a more rapid and more even drainage of the digesting liquor and wash water is brought about. Furthermore, the process enables the rapid removal of the pulp under conditions permitting uniform operation of a discharge pump.

A feature of the invention is the provision of a special construction by which a continuous circulation of a portion of the contents of the pit may be brought about and a more or less constant discharge of liquid over the drainage surfaceis maintained. The special equipment provided for this purpose enables the maintenance of the uniform operating conditions mentioned. The circulating system may conveniently be so connected with the blow pit as to provide for iiexibility of operation to suit varying conditions.

Other features and advantages of the invention will appear from the detailed description of certain illustrative apparatus and methods of operating the same which will now be given in conjunction with the accompanying drawings, in which:

Fig. 1 is a view partly in elevation and partly in vertical section through a blow pit embodying features of the invention and showing a portion of a connected dlgester.

Fig. 2 is a view partly in plan and partly in horizontal section through the apparatus of Figure 1.

Fig. 3 is an enlarged vertical, sectional view through a portion of the wall and drainage surface of the pit.

Fig. 4 is a detail, sectional view along the line 4-4 of Figure 3. f

Fig. 5 is a sectional view, similar to Fig. 3, showing a modified form of construction.

Fig. 6 is a detail, sectional view on the line 68 of Figure 5.

Fig. 7 is a detail, horizontal sectional view through a portion of a pit showing a furtherk modified construction.

Fig. 8 is a vertical sectional view taken adjacent the wall of a pit embodying the construction of Figure 7. Figure 9 is an end elevation of a pump forming part of the preferred equipment. and Fig. 10 is a sectional view through the pump taken 75 along the line 10-10 of"Figure 9.

In the drawings the improved construction, adapted for the new method, is shown as applied to a blow pit'of elliptical form, such as disclosed more fully in my Patent No. 1,840,774, granted 80 January 12, 1932. While the improvements are particularly well suited for application to a pit of this type, they may, if desired, be applied to advantage to other forms of blow pits. A pit of elliptical form may, without the special provisions B5 contemplated by the present invention, be operated with a high degree of efliciency and in such manner as to produce relatively uniform results. However, by the application of the improved features of this invention even greater eiiiciency and uniformity of operation are made possible.

Referring now to Figures 1 to 4 of the drawings, there is shown a digester 10, of any conventional construction, and a blow pit 11, preferably elliptical in form, both carried by a concrete or similar foundation 12. Pulp which has been. cooked and digested in a suitable way in the vessel 10 is discharged though a line 13 into the blow pit 11. A valve 14 in this line controls the blowing operation. As shown in Figures 1 and 2, 100 the pulp is preferably blown into the pit substantially tangent to one of the longer sides and at a point well toward the topof the pit. If desired it may be discharged against va target such as shown in my prior patent so as to effect an even distribution throughout the pit. However, as will be made more apparent in the description of the preferred mode of operation of the system, the provision of a target is not essential.

The pit which is preferably formed of wood,

gesting liquor, should be of lliquid-tight vwhen the pulp density is below 4%.

factory resistance to the action of the spent diconstruction and sufficiently sturdy to withstand the forces of pressure from within or resulting from the creation of a partial vacuum. The bottomv 15 of the pit may be supported by timbers,

not shown, and by means of grooves formed in the side walls adjacent the bottom.

On the bottom 15 there is provided a series of cross timbers 16, laid fiat, and adapted to support a series of longitudinal timbers 17. A second series of cross timbers 18 rests upon the timbers 17 and these are spaced closely enough to support the edges of drainage tiles 19. These tiles, as explained in my prior patent, are arranged to cover substantially the entire bottom of the pit and are provided with a large number of drainvage openings permitting the passage of liquids.

The spaces between the tiles are filled by wooden wedge members 20 secured in any suitable way to the timbers 18 and thus holding the tile bottom in place. It will' be apparent that a liquid storage space of. considerable capacity is provided between the tiles 19 and the bottom 15, this space being only partiallyoccupied by the various timbers. 1

Adjacent one end of the pit, as indicated at 21 in Figure 2, there is provided a sump which is vwalled off from direct communication with the space just mentioned but is adapted to receive the material from above the drainage surface. Associated withvthis sump is a discharge pipe 22 connected with a pump 23, which may suitably be of the centrifugal type, driven by a motor 24.. The pump discharges into-a pipe 25 connected through a T with two pipes 26 and 27. These are provided with valves 28 and 29, respectively, so that the material withdrawn from the sump may be handled in either of two ways or partly in each way. As' will be explained later athird line for the disposal of this material is also provided. The pipe 27 may be connected with any suitable storage tank, not shown; or with riiiiers or any other equipment for further treatment or disposition.

Referring to Figures 9 and 10 there is shown a preferred construction of a pump of the centrifugal type particularly suitable for the purpose of handling the pulp and other material withdrawn from the sump. In order that the pulp may be readily handled by a centrifugal pump it is important that it should be of suinciently low concentration to permit easy flow.

If the pulp becomes too dense it is dicult to build up the necessary velocity of movement. For this reason it is considered advisable to maintain the pulp density below 5%, i. e. the mixture of pulp and water or liquor should contain less than 5% of the, actual pulp fibres, In fact' most. efficient pump operation isprovided As will be f lexplained more fully hereinafter special provisions are made for adding water to the pulp at. the pump or in advance of it to maintain a suitable concentration. To enable the pump to handle the pulp eiciently at densities of as much 'as 5% and possibly somewhat higher the impeller is provided with greater clearances at various points than are normally provided. The pump casing 230 may be of usual form and may have the usual inlet port 231 in line with the axis of the impeller 232. This is carried by a shaft 233 suitably journalled and driven by the motor 24. The impeller is provided with a series lng faces, preferably ycvcloidal in form, adapted to'build up the velocity of movement of the pulp from Zero at the center to a maximum at the blade tips with a constant acceleration. At the tip 235 of each blade the full width of the blade should be maintained so as to provide full power for throwing the stock at maximum speed at this point. It will be noted that the blades are cut awaysatthe tips as well as at the sides, as at points 236, to provide sufficient clearance for the passage of foreign material such as pieces of the digester lining, thus avoiding possible breakage of the blades. a centrifugal pump of this character other means, such as a screw impeller or the like, may be used if it is found necessary or desirable to handle a denser pulp mixture.

Any material which is diverted into the pipe 26 is conveyed to a channel 30 which follows the interior surface of the wall of the pit substantially completely around the same, at a point just above the drainage surface. This channel may, as shown in Figures 3 and 4, be formed between the vertical wall of the pit, the drainage surface. and a curved member 31. This channel member may be cast or formed into a suitable shape, such as a quarter-circle in cross section, or any other appropriate form and may be in one continuous unit or in a plurality of separate sections. It should be formed of material which is resistant to the action of the digesting liquor and is preferably made of hard lead, bronze, .ead coated iron or steel or similar materials. If desired it may be molded of tile or carbon or like substances. It is provided with a vertically eX- tending flange portion 32 having eongated slots 33 adapted to receive screws or bolts 34 by which the channel member may be adjustably secured tervals. By the arrangement disclosed an opening 36 of desired height orwidth may be provided at the inner edge of the channel just above While it is preferred to employ.

the drainage surface and around substantialy its l entire periphery. The end 37 of the channel may be blanked off so as to enforce the discharge of the material, introduced into the channel, through the opening '36 or it may be left open to permit the return directly to the sump of any excess material not discharged lateraly through the opening 36. In any case it will be apparent that suicient back pressure will be developed to force a sheet or stream of the fluid material inwardly over the tile surface.

In Figures 5 and 6 there is shown a modified channel construction which functions in substantially the same way. In lieu of providing a continuous, channel member, there is merely provided a series of spaced cast or forged brackets 38, preferably formed of a noncorrosive metal having elongated slots 39 adapted to receive screws or bolts 40 by which they may be adjustably secured to the wall of the pit. The

of, say, 45 to the wall l ber 41 is held spaced from the drainage surface to an appropriate extent by the screws 40 and by wedge elements 42.

A further modification of the channel structure is illustrated in Figures 'I and 8. As here shown, it may consist of a series of intertting or interconnecting sections 43 of substantially arcuate form, each having complementally constructed ends. Oneend of each section may be provided with an offset flange 44 adapted to engage over a straight extension at the end of an adjacent section in bell and spigot fashion. This construction will allow for inequalities in length and variations in the curvature of the sides of the tank and will also provide for expansion and contractiony of the channel members. The joints formed between the sections may be sealed, if desired, in any suitable way. The general contour of the sections 43 in cross section may be arcuate, similar to the member 31, or it maybe simply provided with a fiat slope, similar to the member 41. Upstanding ears or projections 45 with vertically elongated slots 46 may be provided to permit adjustable attachment of the sections to the wall of the pit. Wedges 47 may also be provided to assist in the support of the lower, inner edges of the sections. It will be apparent that in any of the ways mentioned there is provided a channel around the interior surface of the pit adjacent the drainage surface with an opening of variable size, or possibly a series of openings, facing toward the center of the pit. This, as will be explained more fully hereinafter, assists in various operations of the pit and is particularly helpful in the rapid removal of the pulp from the pit.

A third line 48 yconnected with the discharge side of the pump is also adapted to receive al portion, or all, of the material handled by the pump under control of a valve 49. This line terminates at substantially the center of the pit and has its discharge end 50 practically flush with the drainage surface. As will be explained more fully hereinafter, it may be found desirable at times to so regulate the valves 22, 29 and 49 as to direct all, or a part, of the pump discharge into the center of the pit at the drainage surface. This is particularly desirable for the removal of the final masses of pulp which may have a tendency to accumulate adjacent the center of the pit.

Above the pit there is provided a water trough 5l adapted to receive a supply of water from any convenient source and discharge it,` whenever desired, through aline 51a into the top of the pit. A gate 51b may be provided to control the introduction of water in this way. Parallel with the trough there is provided a water line 52 which may be connected byl a branch line 52a with the intake of the pump 23. A valve 52b may control this connection to the pump. A line 53 connected .by a branch line 54 with the top of the pit may be used to introduce spent, or partially spent, liquor into the pit under control of a valve 55. An outlet 56, connected with a pair of branch lines 57 and 58 controlled by valves 59 and 60, respectively, is in communication with the drainage space, previously mentioned, between the tiled surface and the bottom 15 of the pit.v Through these' connections the spent liquor may be discharged into a rich liquor line 6l to be delivered to any suitable point for conversion into various by-products or for further use, vwith or'without concentration, as by return to the same or a different pit through the line 53. After the rich liquor has i been removed from the pulp,the wash water may be diverted into a-waste liquor line 62 by proper manipulation of valves 59 and 60 and may be dumped into a sewer or disposed of in any other suitable Way.

Having thus described the arrangement and functions of the various portions of the apparatus, a brief explanation will now be given -of severall modes of operation lwhich will be found particularly effective. Let it be assumed now that the pit has been emptied after a previous operation and is ready to be put into use for the draining and washing of a batch of pulpwhich has just been cooked in the digesterlO. The valves 59 and 60 shouldbe closed and the pump 23 left idle; if desired the valve in the line lead. ing from the sump to the pumpmay also be closed. Spent sulphite liquor may now be introduced, by gravity or by means of a pump, through the pipe 54 passing through the top of the pit. This liquor may be taken from any suitable storage or may be passed directly from one blow pit of a series to another. A suiiicient y quantity should be introduced to carry it to a level of, say, 24 inches above the surface of the drainer tiles. A portion ofthe liquor will, ol course, pass through the drainer surface and will fill the space between this surface and the bottom of the pit. However, due to closing of the valves 59 and 60, it will not be discharged. The valve in the line 22 and the valve 28 may now be opened and the valves 29, 49 and 52b closed. The pump may then be set into operation to draw a portion of the spent liquor from the sump 21 a'nd circulate it through the channel 30. This will be distributed through the opening 36, on the inner side of the channel, along the drainer surface and will gradually work back to the sump to be circulated again. After this circulation of the liquor has been started the blow oif valve of the idigester may be opened and the blowing of the pulp and liquor into the pit may be carried on simultaneously with the continued circulation. This will result in a very even distribution of the pulp over the entire drainage area of the pit and will bring about a more even mixture of the pulp and liquor than has heretofore been attained. The continued circulation of the spent liquor, furthermore, tends `to keep the opening in the side of the channel clear so that it may function properly in the later stages of the operation.

After the Icontents of the digester has been blown into the pit, the operation of the pump 23 may be stopped and the valve 60 may be opened so that the spent liquor which has accumulated in the bottom of the pit below the drainersurface, together 'with the rich liquor which is drained from the blow pit, may be passed into the rich liquor disposal line 61. If desired, the spent liquor which has previously accumulated beneath the drainer surface may rst be discharged into the line 62 by opening the valve 59 but inasmuch as this liquor contains certain valuable ingredients, it is considered preferable to pass all of it into the line 6l. It may be kpassed to suitable storage for use in the preliminary stages of a further blowing operation or may be converted in any suitable Way into desired by-products. During the draining period, in which the richest liquor is removed from the pulp, water may be added through the line 51a at the top of the pit. This water gradually drains through the pulp and assists in washing the spent liquor into the line 6l. When the liquor has become so diluted by the addition of such a large quantity of water through. the lpipe 51B in proportion to the liquor contained in the pulp, the valve is closed and the valve 59 opened so that the dilute mixture may be dis- As the next step in the operation, the valve 52h' may be opened while the valves 29 and 49 are kept closed. The pump will then be started and fresh water, or white water, will be drawn into the suction side of the pump together with the waste liquors which have accumulated in the sump 21. This mixture will then be discharged from the pump under pressure into the channel 30 and will be spread, at least in part, over the surface of the drainer tiles. Eventually it will be returned in part to the sump S and may be recirc'ulated. A portion of the water and impurities removed from the pulp will pass through the drainer tiles and will be discharged through the line 57 into the waste liquor line 62. At this stage of the operation, however, the waer will be added to the pulp at a more rapid rate than it is withdrawn through the draining surface. As a result the pulp will be gradually diluted and oated and will be more thoroughly washed. Due to the forcing of the wa er upwardly under pressure through the pulp there will be a tendency to separatethe fibres and lighter mass instead of compacting it as is usually the cas'e when the wash vwater is directed downwardly against the pulp.

Immediately prior to this stage of the process a certain amount of air will have accumulated be.- neatli the drainage surface and as the wash water passes through the openings in this surface, a portion of the air will be displaced and will producev an ebullition through the pulp mass. Due to the even distribu' ion of the pulp and vthe tendency to float the same, a substantially uniform bubbling action will take place throughout the pulp mass. rThis bubbling action will result in a desired agitation of the pulp and will bring about a more thorough washing which will be quite uniform at all points of the pit. It will be apparent that as the wash water circulated through the channel 30 is discharged against the drained pulp mass, it will gradually wear'away the edges of this mass and work its way 'toward the center of the pit. As the pulp is thus gradually undermined and diluted, and floated to a certain exl tent, a very inimate and rapid contact will be provided between the water and the fibres of the pulp stock. This will result in a considerable speeding up of the washing action and will accordingly bring about an economy in the operation of the pit.

After the washing operation has been coninued for a sufficient period of time, depending upon the slowness ofthe stock, the pulp may be removed from the blow pit and passed to storage or to any desired further step in the treating process. At this time the valves 59 and 60 will both be closed f. and the valve 29 will be opened while the valves 52b and 28 will be partially closed. By proper adtion of the material discharged by the pump may be passed to storage and the remaining portion may be returned to the pit through the channel 30. This regulation of the circulation of the pulp handled by the pump enables the operator to exercise a very denite control over the uniformity of the mixture, as the mass of pulp and water is removed from the blow pit. 'I'he condrained through the drainer surface.

tinued introduction of the fresh water through the line 52n may be regulated by the valve 52h' This water will in effect replace the water which continues to drain through the tile surface. If desired the `valve 59 may 'be left open, or partly open, during this period in the operation so as to permit the discharge of any water which is By continually adding water to the pulp as it is withdrawn and passed to storage, and by maintaining a circulation o f a portion of the mixture handled by the pump at this time, the removal of the pulp is carried out under uniform conditions and is accordingly made more eicient. The circulation of apart of the mixture assists in washing the body. of pulp toward the sump and insures the maintenance of an ample supply of material in the sump at all times. When the greater portion of the pulp has been removed, and there remains only a comparatively small portion at the center of the pit, the Valve 28 may be closed and the valve 49 opened. This will cause a poriion of the mixture discharged by the pump to/be passed into the center of the pit through the opening 50 located at the level of the drainer surface. The material discharged at this point will then assist in Vwashing the remaining portion of the pulp toward the sump. If desired the valve 28 may be left partially open at this stage of the operation so that the flnal the apparatus, during the several stages,`in the4 manner explained are numerous. A more thorough mixture of the pulp and wash water is broughtabout with the result that a more efficient Washing action is produced. At the same time, due to this greaterefciency, a more economical use of fresh water is made possible. The operation of the blow pit is speeded up in View of the fact that the drainage action is made more uniform over all portions of the pit and a more rapid drainage of the water from the pulp is permitted. The final mixture o f pulp and water is maintained at a more uniform consistency than in any prior blowpit operations so that it may, if desired, be passed directly 'to the next stage in thernanufacture of paper or the like. Furthermore, the loadplaced upon the pump 23 is made more uniform throughout the operation due to the more uniform consistency ,of the pulp and due to the fact that the sump constantly has a full supply in the sump 21 to draw upon. The resulting increase in the efficiency of the pump permits the use of a smaller pump consuming a smaller amount Of energy.

Due to the continuous delivery of a comparaout the removal of pulp from the pit, it is possible to employ any suitable type of consistency regulator to advantage in the line 2'7. For example any known form of density responsive device such as a motor driven resistor. or control unit 63 may be employed at a point adjacent the rifiiers or other equipment when further treatment is to take place. This unit, in a well known manner7 may then control the operation of a motor driven Valve 64 in a water line 65 adapted to introduce water into the discharge line 27. Preferably the water will be introduced as near as possible to the valve 29 so that uthe pulp may become uniformly diluted by the time it reaches the control unit 63. It will be apparent that by this arrangement, so long as the desired density at the unit 63 is less than the minimum density of the pulp delivered through the valve29, it may be brought to and maintained at a uniform consistency by the introduction of a regulated amount of water through Athe line 65.

If desired the apparatus disclosed may be operated in a somewhat different manner. In lieu of starting the operation by filling the blowpit with spent liquor, a quantity of fresh wateror white water may be introduced prior to the blowing operation. For example, all valves may initially be closed and the gate 51b controlling the discharge of water from the trough 5l into the branch 511 may be opened to discharge a quantity of fresh waterinto the pit. Sufcient water may be introduced, for example, to carry it to a level between 3 and 4 feet above the drainer surface before the gate 51b is closed. As in the rst operation, a portion of this water will drain through the tile surface and will accumulate in theI space between this surface and the bottom of the pit. The valve in the line 22 may now be opened along with the valve 52b and the pump discharge valve 28, the other valves of the system remaining closed. The pump is now set into operation so as to produce a circulation of the water from the sump around the channel 30 back to the pit for re-circulation. The blow-off valve 14 may now beopened and the circulation of the water, in the manner explained, may be continued throughout the blowing operation. The result of this circulation will be substantially the same as the circulation of the spent liquor in the first operation. When the entire contents of the digester has been blown into the pit. the operation of the pump may be stopped. The valve 59 may then be opened to discharge the hot liquors from the pulp, together with water previously in the pit, to the sewer. The gate Sib may be partially opened, if desired,

during this draining operation so as to assist,

in washing the liquor from the pulp and forcing it into the sewer. When this drainage operation has been completed, the pump is again started into operation and water received from the line 52 is circulated through the channel 30 with the same result as in the rst method of operation explained. It will gradually wash away, dilute and float the pulp and by the bubbling or ebullition action previously explained, will provide a very intimate Contact between the water andv the pulp fibres. After the washing action has continued for a sufficient period of time, it may be discontinued and the pulp may be removed from the pit in the same manner as explained in connection with the rst method of operation.

While the methods of operaton explained above are considered preferable. it will be apparent that various stages of the operation may be varied to suit requirements. For example. the

circulation of the spent liquor or the white water' during the blowing operation may be eliminated if desired. Furthermore, the pit may be empty when the blow-off valve 14 is opened in lieu of having a layer of spent liquor or white water at Athis time above the drainage surface in the manner explained. n this case after the blowing operation has been completed, a quantity of wash water may be introduced from the trough 51 to assist in forcing the liquor from the pulp and to provide a simple washing action. Spent liquor or white water may, if desired, be introduced into the pit simultaneously with the blowing operation. Other variations of the process may occur to those skilled in the art. In describing a lparticular' form of apparatus and various specific modes vof operation of the equipment, it is not intended to limit the scope of the invention to theseA particular forms. `The various phases of the invention are capable of a variety of modi# cations all fall'ng within the scope of the claims.

What I claim is:

l. A method of operating a blow-pit which comprises continuously withdrawing liquid from the pit and returning at least a portion of it to the pulp in the pit. l

2. A method-of operating a blow-pit which comprises continuously withdrawing liquid from the pit and spreading at least a portion of it over the drainage surface of the pit.

3. A method of operating a blow-pit which comprises filling the pit with a liquid to a level above the drainage surface, blowing the contents of a digester into the pit in the presence of said liquid, draining the spent liquor from the blown material, and continuously recirculating a washing medium in intimate contact with the pulp to effect a thorough washing thereof.

4. A method of operating a blow-pit which comprises filling the pit with a liquid to a level above the drainage surface, blowing the contents of a digester into the pit in the presenceof said liquid, and maintaining a continuous circulation of a portion of the same liquid during the blowing operation.

5. A method of operating a blow-pit which comprises spreading a film of liquid over the drainer surface of the pit during the blowing of pulpfrom a digester into the pit.

6. A method of removing pulp from a blow-pit which comprises continuously directing a liquid .against the edges of the pulp mass adjacent the drainer surface to float the pulp and wash it into a sump formed in the pit.

7. A method of removing pulp from a blow-pit which comprises continuously returning to the pit a portion of the pulp withdrawn to assist in flushing the pit while passing another portion to storage.

8. A method of operating a blow-pit which comprises introducing spent liquor into the pit to a level substantially above the drainer surface,

lowing the contents of a pulp digester into the pit, continuously agitating the spent liquor as the pulp is being blown, and draining from the pit said spent liquor and the rich liquor from the pulp.

9. A method of operatingv a blownpit which comprises introducing spent liquor into the pit to a level substantially above the drainer surface, blowing the contents of a pulp digester into the pit, continuously circulating a portion of the spent liquor during the blowing operation and draining from the pit said spent liquor and the rich liquor from the pulp.

l0. A method ofremoving pulp from a blcwpit which comprises continuously returning tothe pit a portion of the pulp withdrawn to assist in flushing the pit while passing another portion to storage, and continuously adding-water to the pulp withdrawn from. the pit.

li. A method of removing pulp from a blow-pit which comprises contnuously withdrawing a pulp mixture from a sump formed in the pit, continuously directing a portion of said mixture against the pulp remaining in the pit to flush it towardthe sump, andrcontinuously passing a portion ofthe withdrawn mixture tostorage.

l2. A method of removing pulp from a blowpit which comprises continuously withdrawing a pulp mixture from a lsump formed in the pit, returning a portion of said mixture-to the pit and directing it against the edges and center of the pulp mass vin the pit, and continuously passing a portion of the withdrawn mixture to storage.

13. A method of removing pulp from a blowpit which comprises continuously withdrawing a pulp mixture from a sump formed in the pit, continuously directing a portion of said mixture against the pulp remaining in the pit to flush it toward the sump, adding water to the pulp as it is being withdrawn to maintain a substantially uniformconsistency, .and continuously passing a portion-of the withdrawn mixture to storage.

14. A method of operating a blow-pit which comprises partially filling the pit with a wash liquid to a point abovey the drainage surface, blowing pulp from a digester into said pit while maintaining the drainage valves closed, circulating a portion of said wash liquid during the blowing operation, subsequently draining the pulp, then adding a washing agent to the pulp, and maintaining a continuous circulation of a mixture of pulp and said washing agent during the washing operation.

15. Apparatus of the class described comprising a vessel having a foraminous drainage surface and a retaining wall, liquid conducting means extending along the interior surface of said retaining Wall and adapted to discharge a liquid toward the center of said Vessel, and means for withdrawing material from said vessel and returning it thereto through said conducting means.

16. Apparatus of the class described comprising a. vessel having a foraminous drainage surface and a retaining wall, liquid conducting means extending along the interior surface of said retaining wall adjacent said drainage surface and adapted to spread a liquid over said surface, and means for withdrawing material from said vessel and returning it thereto through said conducting means.

17'. A blow-pit having a drainage surface and a curved retaining wall, a channel'secured to the interior surface of said wall and extending substantially around the pit, said channel being constructed and arrangedto discharge a uid substantially throughout its length 'toward the center of the pit,v and means for discharging a fluid medium into said channel.

18. A blow-pit having a drainage surface and a curved retaining wall, a channel secured to the interior surface of said wall adjacent the drainage surface, said channel being adapted to discharge a fluid over the drainage surface, and means for introducing a fluid medium into said channel.

19. A blow pit having a drainage surface and a curved retaining wall, a channel extending" around the interior of said retaining wall adjacent said drainage surface, said channel being adapted to discharge a fluid over said surface, and means for withdrawing a iiuid mixture from'said pit and returning a regulated portion thereof through.'

said channel.

20. A blow pit having a drainage surface and a curved retaining wall, a channel extending around the interior of said retaining wallfadjacent said drainage surface, said channel being adapted to discharge a fluid over said surface, means for withdrawing a iiuid mixture from said pit and returning a regulated portion thereof through said channel, and means for introducing water into the mixture returned to the pit. i

21 In a blow-pit a drainage surface, a curved retaining. wall, a channel following the interior contour of said wall, and means for adjustably supporting said channel adjacent the drainage surface with an open side facing said surface.l

22. In apparatus of the class described a blowpit, means for withdrawing a pulp mixture from said blow-pit, means for returning a portion of said mixture to the blow-pit, means for steadily. delivering a portion of saidv mixture for further disposition, and a consistency regulator associated with said last mentioned. means.

23. A method of handling pulp withdrawn' from a blow-pit which comprises continuously withdrawing the pulp, returning a portion of the pulp 'to the pit, passing another portion in a steady 'stream for further disposition, and adding a 

